This invention relates to power supply units, and more particularly to a power supply unit for use in an electric discharge machining apparatus.
FIG. 1 is a circuit diagram showing an example of a conventional power supply unit for an electric discharge machining apparatus. Referring to the figure, reference numeral 1 designates a first DC power supply circuit which produces a first variable DC voltage signal; 2, an interelectrode gap formed between a workpiece to be machined and an electrode; 3, a first switching element for switching the DC voltage signal supplied from the first DC power supply circuit 1 to the gap 2; 4, a first drive circuit for driving the first switching element 3; 5, a current limiting resistor for limiting a current flowing from the first 1 to the gap 2. The first DC power supply circuit 1, the first switching element 3, the first drive circuit 4, and the current limiting resistor 5 constitute an auxiliary switching circuit. Reference numeral 6 designates a second DC power supply circuit which produces a second variable DC voltage signal higher than the first variable DC voltage signal; 7, a second switching element for switching the second DC voltage signal; 8, a second drive circuit for driving the second switching element 7. The second DC power supply circuit 6, the second switching element 7, and the second drive circuit 8 constitute a main switching circuit. Reference numeral 9 designates a detector for detecting a voltage appearing across the interelectrode gap 2 to thereby detect the state of the gap. In response to the detection result, the detector 9 operates to control the first drive circuit 4 and the second drive circuit 8 on the basis of an internally programmed set of sequences; and 10 and 11, diodes for preventing the reverse flow of current.
An operation of this power supply unit will be described. When the first drive circuit 4 constituting the auxiliary switching circuit causes the first switching element 3 to be turned on, the first DC voltage signal is applied to the interelectrode gap 2 through the current limiting resistor 5 and the diode 10.
As described above, the detector 9 detects the gap voltage appearing across the interelectrode gap 2 so that the detection result is applied to the first drive circuit 4 and the second drive circuit 8 as a control signal. More specifically, the detector 9 is able to detect three types of gap states so as to supply as a control signal to the first and second drive circuit 4 and 8 an internally programmed sequence signal selected according to the detected type of states. These three types of gap states will be described in detail. When the gap 2 is in an open state as a first state, the first switching element 3 is turned on, as shown in FIG. 2(a), to allow the output of the first DC power supply 1 to be applied to the gap 2. Since the gap 2 is maintained open, the voltage detected by the detector 9 is equal to the output voltage of the first DC power supply circuit 1 as supplied as shown FIG. 2(c). Thus, upon detection of the first state, the detector 9 controls the first drive circuit 4 according to a preprogrammed sequence to thereby cause the first switching element 3 to maintain its on-state.
Then, upon an occurrence of discharge at the gap 2, a discharge current shown in FIG. 2(d), which is limited by the current limiting resistor 5 flows resulting in drop of the voltage at the gap 2 as shown in FIG. 2(c). Upon detection of the voltage drop at the gap 2, the detector 9 judges that the second state is present, as a result of which a predetermined sequence control is then executed. More specifically, the auxiliary switching circuit is provided with the current limiting resistor 5, and therefore a sufficient amount of discharge current is not allowed to flow therethrough. Consequently, the detector 9, upon detection of the second state, starts controlling the second drive circuit 8 based on the above-described sequence control to thereby turn on the second switching element 7 as shown in FIG. 2(b) so that the output of the second DC power supply circuit 6 is applied to the gap 2. Then, a large current flows through the gap 2 as shown in FIG. 2(d) for the reasons that the output voltage of the second DC power supply circuit 6 is higher than that of the first DC power supply circuit 1 and that there is no current limiting resistor on the output side of the second DC power supply circuit 6. As a result, it becomes possible to carry out the electric discharge machining operation with a maximum current to be effected. Upon detection of the second state, the detector 9 also controls the first drive circuit 4 so that the first switching element 3 will be turned off with a delay of a predetermined time as shown in FIG. 2(b). The period in which the second switching element 7 maintains its on-state corresponds to a discharge period specified by a predetermined sequence control.
The conventional power supply unit for electric discharge machining apparatus thus constructed allows only a potential of a predetermined polarity to be applied between the workpiece and the electrode. This induces not only electrolytic corrosion and electrolysis which promotes to wear the surface to be processed but also the electromagnetic effect derived from the application of potentials of a single polarity which then encourages to magnetize it, thus causing the problem of requiring time-consuming post-treatments after the processing, and the like.